borderline dissorder and depression

well.. suffer from the same dissorder and i know what you're feeling.. have you tried seeing a psychiatrist? maybe some medication to be more stable in your relationships? maybe that would be a good idea.. and if you don't want.. well it's a matter of selfcontrol. look for the reason why you screw things up, there's always a reason. In my personal experiece, i used to screw things up coz i was used to unstable things and relationships.. so when i was feeling "too well" i needed some mess. It was something unconsious, i just couldn't enjoy when everything was ok. now i'm learning to do it and i can tell you it feels great. it takes a huge effort but it's possible. I stopped taking my medication and cutting myself, and i just have semanal sessions with my shrink. It really helps me.
you just need to control your emotions with your head. no self-pity and just rationalize things. Remember you'll feel even worst if you lose the person you love.